1. Field of the Invention
The invention relates to a method of transmitting a data stream comprising subsequent groups of N symbols, N being an integer, said method comprising, for each of said groups, the step of modulating each carrier of a frequency division multiplex of N carriers with a distinct symbol from this group for a period exceeding the periods between subsequent groups, such that the modulation signals of said carriers have a symbol rate at least N times lower than the symbol rate in the data stream and that inter-symbol interference in these modulation signals is deliberately introduced.
The invention also relates to a transmitter and a receiver for cooperation with said transmission.
2. Description of the Related Art
A method of this type is known from U.S. Pat. No. 4,641,318 by Addeo. The known method inter alia provides the possibility of reducing the error probability of data transmission via a channel impaired by Rayleigh fading. According to this method, the symbol period in the modulation signals of the frequency multiplexed careers is rendered larger than the duration of a Rayleigh fading which is characteristic of the channel. It prevents that this fading affects all the information in the transmission signal on a symbol or a sequence of symbols, which may cause irreparable errors in the data received.
The known method basically proposes a combination of two measures for increasing the symbol period in the transmission signal. Firstly, a data stream I having a relatively high bit rate is converted into N parallel sub-data streams at a relatively low bit rate. The symbol period in the sub-data streams is N times Ti, the symbol period in the data stream I. Secondly, the symbols in the sub-data streams are spread over an even longer period than N times Ti. This is effected by time-continuous filtering of the sub-datastreams, such as to smear out the symbol energy in time. The filtered sub-data streams are applied to a frequency multiplex modulating arrangement, such that each of these streams modulate a distinct carrier. Effectively, each symbol of subsequent groups of N symbols in the data stream, modulate a distinct carrier for a duration which exceeds N times Ti.
The resulting transmission signal is a frequency multiplex of data modulated carriers in which the symbol rate is N times lower than that of the data stream, but having inter-symbol interference (ISI) between time-adjacent symbols in the modulation signal of each carrier. This ISI is deliberately introduced at the transmitter side in view of reducing the error probability of the data transmission. This may seem contradictory. However, at the receiver side, the deliberately introduced ISI may be compensated by a suitable chosen form of equalization and/or symbol detection. The basic idea is thus to pre-distort information at the transmitter side and to de-distort it at the receiver side, such as to make the transmission signal less vulnerable to impediments of the transmission channel.
The method proposed by Addeo has the drawback that its implementation requires many components and is consequently expensive and voluminous. The frequency multiplex modulator comprises N modulators with a different carrier frequency for each modulator, preceded by N filtering devices. The modulators receive analog signals, or at least signals in which a considerably larger number of signal levels can be distinguished than in the sub-dam streams at the inputs of the filtering device. A system of N analog modulators for such input signals cannot easily be realized.
Notably, for transmitting data at high bit rates through channels having large delay time differences, the proposed method is not very suitable. An example is terrestrial transmission of video data at a rate of, for example 20 megabits per second, in a mountainous environment, with delay time differences of the order of 10 microseconds. A factor N of approximately 1000 at a minimum is then required to sufficiently increase the symbol period with respect to the characteristic duration of a Rayleigh fading.